In the field of high-speed optical communications, optical modules such as a 100 Gbps (4×25 Gbps) module with a transmission distance of 10 km are usually implemented by four electro-absorptive modulated lasers. Therefore, the optical module includes multiple laser direct-current bias driving circuits, and each driving circuit controls an output optical power of a respective one of the four electro-absorptive modulated lasers. For a driving circuit, the main challenges are to realize low power consumption and small size.
FIG. 1 is a circuit diagram of a conventional laser driving circuit 100. The conventional laser driving circuit 100 is characterized by the following.
First, the laser driving circuit 100 includes one or more automatic optical power control loops. In each loop, an output optical power of a laser 110 is detected by a photodiode 112, and a photoelectric current Impdk (k=1, 2, . . . , n) is generated accordingly. Then, an I/V converter circuit 114 is used to generate a feedback voltage signal Vfbk (k=1, 2, . . . , n) that is proportional to the laser's output optical power. The feedback voltage signal Vfbk is fed into a controller 116, in which the feedback voltage signal Vfbk is compared to an optical power setting voltage Vpsetk (k=1, 2, . . . , n) to obtain a deviation signal, which is processed by an integral control calculation to obtain a control voltage Vctrlk (k=1, 2, . . . , n). Furthermore, a V/I-converter laser current driving circuit (#1, #2, . . . or #n) 118 is used to convert the control voltage Vctrlk to a corresponding laser driving current Ibiask (k=1, 2, . . . , n).
Second, the laser driving circuit 100 uses a direct-current fixed power source to supply a working voltage Vin for one or more V/I-converter laser driving circuits.
FIG. 2 is a circuit diagram of a typical V/I-converter laser current driving circuit 200. In the V/I-converter laser driving circuit 200, a bipolar junction transistor (BJT) or field-effect transistor (FET) 202 is used on an output end to drive a load of a laser 204. An output loop of the V/I-converter laser driving circuit 200 includes a current detection amplification circuit 206 that detects a current to generate a negative feedback voltage signal which is proportional to the detected current. The negative feedback voltage is compared with the control voltage signal Vctrl. An operational amplifier 208 is used to realize the V/I conversion.
A problem with the conventional laser driving circuit is that a relatively high output voltage V+ needs to be configured for the fixed power source to meet the operating requirements of all of the lasers in an optical module, because the operating voltage of each laser varies significantly from batch to batch and changes with the output power setting. A relatively high output voltage leads to relatively high power consumption, of which a large portion is wasted in the driving circuit, and thus power consumption by the driving circuit is not optimized. This is because: V+=Vdrop+Vld, where Vdrop is the voltage drop generated by an output driving transistor, e.g., transistor 202, and current detection amplification circuit, e.g., circuit 206. In order to ensure the proper operation of the driving circuit and obtain good linearity in the V/I conversion, Vdrop usually needs to be 0.5 V or above (i.e., Vdrop(min)=0.5 V). Vld is the laser's voltage drop. Vld=Vld0+Ild*Rld. Vld0 is the laser's breakdown threshold voltage (usually about 1 V), Rld is the laser's direct-current internal resistance (usually 8-15 ohm), and Ild is the laser's current (usually 0-100 mA). Rld increases as the laser's operating temperature increases, and Ild changes with the laser's output power setting.
From the perspective of circuit design, in order to ensure that all lasers can operate over the entire operating temperature range, the output voltage of the direct-current fixed power source must be set to be no lower than V+(min)=Vdrop(min) +Vld(max). The ranges for the parameters above are known, for example, V+(min)=0.5V+(1.0V+100 mA*15 ohm)=3.0 V. In such a circuit configuration, for lasers with low internal resistance and high efficiency, such as one where Rld=8 ohm and Ild=50 mA, Vld=1.4 V, Vdrop=V+−Vld=1.6 V, and the laser's power consumption Pld=Vld *Ild=1.4 V*50 mA=70 mW, and the wasted power Plost=Vdrop*Ild=1.6 V*50 mA=80 mW. In particular, high-speed optical modules such as an 100GLR4 usually have at least 4 channels of lasers, so the total power wasted reaches 80 mW*4=320 mW, which cannot be neglected if the target power consumption of the high-speed optical module is expected to be kept under 0.5 W.